In 2019, the site was on its way to becoming a mixed hardwood stand with a very strong oak component.
Nine growing seasons after seeding, the site is on its way to becoming a mixed hardwood stand with a very strong oak component. There were about 4200 stems/ acre of red oak present, averaging about 3.5 feet in height in 2019 (see Table 1 at link below). There are also low to moderate numbers of other hardwoods species present, including black walnut and sugar maple.
Table 1. August 2019 Regeneration Check Results
Seedlings are exhibiting slowed growth due to heavy grass and annual weed competition. Growth should accelerate considerably after crown closure is achieved.
While survival is excellent, growth is slowed due to competition from heavy grass and native plant cover. Growth and vigor should accelerate considerably after achieving crown closure, which should start to happen on most of the site over the next 5 to 10 years.
Seedlings are growing better in some areas than others, but in time the entire field will almost certainly reach a condition of red oak dominated forest, with a good component of other species. It will just take longer (and perhaps an additional vegetative release) before slower-growing portions reach crown closure, and therefore forested condition.
Soil compaction had a negative impact on seedling growth
There are several possible causes of the spatial growth variation of seedlings across the site, including differences in soils, herbicide effectiveness during early grass and weed competition control efforts, and seed sowing rates. Unfortunately, we can’t go back in time to assess spatial differences in early herbicide effectiveness or sowing rates. A potential factor that we can analyze now is soils.
Soils analysis description
Working with Professor Nic Jelinski from University of Minnesota Soils, Water, and Climate Department and Wade Kruger, UMN student and MNDNR intern, we assessed impacts of soil characteristics on establishment and growth of tree species. The following activities were performed: two soil samples collected, 12 tree plots for tree heights, and several seedlings were excavated for root structure examination.
Our original intent was for a more ambitious study plan of a soil sample at each of the twelve tree data plots taken, but we were unable to accomplish that due to overall workloads of the staff involved. While we think a study design of a soils plot at each tree plot could still be a worthwhile goal for a future case study, we do offer one finding from the soils analysis we were able to accomplish. Specifically: Greater soils compaction can result in slower seedling growth.
Soils analysis finding: Greater soils compaction can result in slower seedling growth
While the entire site has excellent distribution of red oak and a mixture of other species, seedlings are clearly growing better in some areas than others, as shown in Table 2 (see link below). For example, the red oaks at plots 2 and 10 averaged 60 inches and 59 inches in height, respectively. In contrast, the red oaks at plots 1,3, 7 and 15 have average heights of 30 inches or less. Plots 8,9,11,12,13 and 14 average from 37 to 47 inches in height. See figure 1 for locations of the tree data plots and Figure 2 for locations of soils plots.
As shown in Figures 3 and 4, we found variation in soil structure and properties on the site. According to Dr. Jelenski, the most significant soils-related factor impacting tree growth and vigor on this site is compaction. A result of excessive compaction in some soils can be impeded root growth, which lowers the ability of seedlings to absorb nutrients and water. In wet conditions, soil compaction can also decrease aeration, which inhibits respiration. Impeded root growth can be seen in the root shown in Figure 5.
Table 2. Average Height and Stems/Acre by Species in June 2020
Figure 1. Locations of 2020 tree data plots
Figure 2. Locations of the two soil samples we took on the seeded field
Figure 3. Comparison of A horizons in 2 soil profiles. Westernmost sample on top and easternmost on bottom
Figure 4. Field notes for two soil samples
Figure 5. Root structure on a red oak seedling from the vicinity of our eastern soil plot
A note about boxelder: Boxelder was not in the direct seeding mix for this site, but seeded in from surrounding trees, which is common on direct seeding sites. Boxelder levels would have been controlled if they were excessively shading and choking out crop trees. However, at moderate levels, such as was the case on most of this site, their presence can be beneficial because they can help accomplish objectives of:
- Earlier “crown closure” to shade out grass competition
- Improved timber quality of crop species by providing side competition to nearby stems and forcing them to grow straighter, and with fewer low limbs.
Note on small patch not direct seeded: For purposes of documentation for anyone that may wish to continue assessing conditions on this site over time, we need to note that there is an approximately ¼ to ½ acre area that was not row-cropped prior to the direct seeding (see Figure 8). It was a bit of a low spot, with pole-sized boxelder on it. The pole-sized boxelder were killed, and black walnut nuts were seeded into in this patch at the time the rest of the area was disced and direct-seeded. This patch currently contains a mix of 10-year-old boxelder and black walnut seedlings that are that are exhibiting growth superior to the rest of the field that had been row-cropped. Superior growth is likely caused by reduced soil compaction and greater moisture retention in this patch.
For ground cover in 2020, we observed mostly high cover of sun-loving native plants, including goldenrod (Solidago sp.) along with some grasses, asters, raspberries, clematis, and wild parsnip.